In general, the afterglow time of a phosphor is extremely short, and the emission of a phosphor rapidly decays when external excitation is ceased. However, in rare cases, even after the cessation of external excitation, some phosphors maintain afterglow of a level perceivable with the naked eye for a considerably long period of time (from a few tens of minutes to a few hours); such a phosphor is referred to as a phosphorescent phosphor or a phosphorescent material in discrimination of such a phosphor from common phosphors.
Examples of the known phosphorescent phosphors include: sulfide phosphors such as CaS:Bi (purple-blue emission), CaSrS:Bi (blue emission), ZnS:Cu (green emission) and ZnCdS:Cu (yellow-orange emission). Any of these sulfide phosphors suffers from a problem such that the phosphors are chemically unstable or poor in light resistance. Even when the zinc sulfide phosphorescent phosphors are used for luminous watches, there are practical problem such that the afterglow time allowing the naked eye to perceive the time shown by the watches is approximately 30 minutes to 2 hours.
Accordingly, the present applicants have invented a phosphorescent phosphor having an afterglow lasting for a far longer period of time than commercially available sulfide phosphors, and further, being chemically stable and excellent in light resistance over a long period of time. Specifically, the present inventors have invented a phosphorescent phosphor comprising, as a matrix, a compound represented by MAl2O4, wherein M represents at least one or more metal elements selected from the group consisting of calcium, strontium and barium, and have been granted a patent (see, for example, Patent Literature 1).
The invention of the aluminate phosphorescent phosphor described in Patent Literature 1 has an afterglow lasting for a far longer period of time than conventional sulfide phosphorescent phosphors. Further, the aluminate phosphorescent phosphor is a phosphorescent oxide phosphor, and hence is chemically stable and excellent in light resistance; consequently, it comes possible to provide a long-afterglow phosphorescent phosphor applicable to various applications.
Additionally, for the purpose of responding to the needs of safety applications in low illumination intensity environments such as indoor type safety signs for escape guidance, the present applicants have proposed a phosphorescent phosphor having a high afterglow luminance even under the low illumination intensity conditions (see, for example, Patent Literature 2).
The invention described in Patent Literature 2 has satisfied the needs under the low illumination intensity conditions such as the indoor conditions. However, there have been demanded phosphorescent phosphors suitable for outdoor applications as alternative applications, in particular, such as applications to information signs, safety signs and guide signs placed outdoors. In other words, demanded are phosphorescent phosphors having a high afterglow luminance after 10 to 12 hours after sunset under the conditions that the excitation due to the sunlight is provided in the daytime up until sunset.
Such use of the sunlight as an excitation source of the phosphorescent phosphors results in the dependence of the illumination intensity on the weather. However, according to the reports on the examination of the matter such as the excitation conditions in outdoor applications, it has been reported that the sunlight can be evaluated as the ultraviolet ray irradiation quantity irrespective of whether the weather is fine or cloudy. For the purpose of achieving a quasi-reproduction of the ultraviolet radiation intensity in the daytime up until sunset, it is recommended that a xenon lamp is used as a light source, and irradiation at an ultraviolet radiation intensity of 400 μW/cm2 is performed for 60 minutes or more and 180 minutes or less (see Non Patent Literature